Calibration Cycles

Calibration frequency will vary with the application

A lot of people play fast and loose with the frequency of calibration on their gages, masters and instruments. But when it comes to gage blocks, they automatically assume that since they are the most accurate physical master usually available, they have to be done on an annual basis.

This approach may be necessary in some cases but not others as I hope these thoughts will clarify for you.

When new, gage blocks are made to very close tolerances but in use they can vary from their new specification for length by a significant amount and still be as useful. This is made possible by the calibration report you’ll receive when they have been re-calibrated. While their “size” may appear to have changed a bit due to wear, as long as you know what their size is, you’re good to go. This is due to the fact that you can adjust for the amount of wear or change when using them by applying the data from the report.

You can use the blocks and ignore the variations noted on the calibration report if those variations are insignificant compared to the tolerances on the gage, instrument or work you are measuring. For example, if you are calibrating micrometers or calipers to ensure they are within 0.001” overall accuracy, a change in the blocks of twenty millionths or more of an inch over time will be of no consequence in the grand scheme of things.

Many people do this but come unstuck when they start specifying what tolerances the blocks have to be within for the application. They get tripped up by specifying the new block tolerance which the blocks will never meet once they’ve been used a few times. What they should be specifying is a maximum tolerance they will allow the blocks to be for use this way. For example, if the set was within ±10 millionths of an inch as new, their process could indicate that blocks within ±25 millionths of an inch are required. This will allow them to ignore minor variations over time.

Maximum use of blocks will be obtained if the calibration process requires their calibrated values to be used. Under this scheme of things, they could be out by 50 millionths of an inch and still be used as masters. However, like all good things, there’s a hitch to this.

The “hitch” is that if the blocks have worn that much, there’s a good possibility they are no longer flat or parallel to such an extent that these changes in their geometry will cause significant variations in calibrated values. To avoid this, ensure all reports are reviewed for such conditions. Like their tolerances in length, you might establish a limit for such conditions in used blocks to be safe and avoid lengthy discussions with an auditor.

Calibration frequency will vary with the application. For example, if the master or block involved is only used once a year, you could get it calibrated every three years or more. But if it is in general use many times a year, you would have to shorten the interval.

All of my remarks are based on typical situations faced by manufacturers. If you are operating a calibration facility where the highest standards are usually required, the game changes to suit. The best way to cope is to always apply the calibrated values of gage blocks or any other masters you are using.

Ensure your blocks and primary masters are kept in pristine condition so you can use my suggestions to extend their calibration frequency by placing some restrictions on their use. One aspect of this is to limit who can use them to only your most skilled technicians. Similarly, limit what they can be used for and lastly, how often they can be used. Back up these requirements by keeping them under lock and key with a calibration sticker that will show if they have been used at a time when they should not have been. This means sealing the box containing them so that the seal has to be broken to get to them.

Another approach is to have their calibration record include who used them for what purpose on what date since there may be an occasion when you may wish to use them as a reference to settle a dimension that is in dispute.

Like all such equipment, care, calibration and control are the keys to getting the appropriate level of precision out of them over time.

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Hill Cox is the chairman for the technical committee for the American Measuring Tool Manufacturers Association and president of Frank Cox Metrology Ltd. He can be reached at hill@frankcox.com or at 905-457-9190.